Police LIDAR (Laser)

What is Lidar (Laser)?

Light Detection and Ranging (Lidar) is the latest speed measuring device used in Virginia. Lidar guns (also known as "laser guns"), are handheld laser devices that law enforcement agents point at cars to measure their speed.

How Does Lidar Work?

Lidar guns measure speed using a "time-in-flight" method. The device projects invisible, infrared laser beams at a rate of about 120 to 238 beams per second for a minimum of 0.3 seconds. The Lidar device then measures the time required for each of beam to bounce back. The device then multiplies the time "in flight" by the speed of light (983,571,072 feet per second) and divides the total distance by two to produce the distance between the target and the gun. The Lidar gun measures the distance of the object 120 to 238 times per second (once for every beam).

Lidar tracks the change in distance during a given period of time and determines the target's speed. For example: if the target began 546 feet away and ended at 510 feet away and 0.3 second elapsed, Lidar will calculate that the object traveled 36 feet in 0.3 seconds and convert that measurement to 81.8 miles per hour.

Which Lidar Devices Do Police Use In Virginia?

All Lidar devices used by law enforcement in Virginia must be approved by the Virginia Division of Purchase and Supply. There are approximately 19 different types of Lidar units that are approved for speed enforcement. However, by far the most popular Lidar units in Virginia are the Prol Lite + and the Pro Laser III (manufactured by Kustom Signals). Both devices are designed and required to measure speed within plus-or-minus one mph.

What is the Range of a Lidar Device?

The range of a Lidar device is well over two thousand feet (more than one-third a mile). However, the functional range of a Lidar device (without out a tripod) is limited by the infrared beam's width relative to the target's size, the reflectivity of the target, atmospheric conditions, the cleanliness and age of the Lidar lens, whether the device is being directed through glass (e.g. the cruiser's windshield), and the operator's ability to hold the device steady.

It is difficult for a law enforcement officer to continuously hold the beam on a moving vehicle that is further than nine hundred feet away. It is also difficult to identify a vehicle in daylight if it is further than eight hundred feet away.

How Wide is a Lidar Beam?

The width of Lidar's infrared beam is typically .003% of the target's distance. For example: if a trooper measures a car that is 1,000 feet away, then the car will be struck by a beam that has a three-foot diameter. The greater the range, the bigger the beam diameter and the greater the difficulty of measuring just one target at a time. Most Lidar devices in Virginia will not produce a valid measurement if a part of the beam is striking the intended target while the rest of the beam is striking an object behind the intended target.

Lidar Sweep Error

Moving the Lidar beam during the measurement process may artificially increase the speed reading. This is called "Sweep Error".

When a police officer's hand moves during the measurement process, the Lidar beam moves from one location to another. The difference in distance between the two locations may be read as if the device was measuring one moving object. The further the operator is from the target the more likely sweep error is to occur.

A common type of sweep error occurs when an officer directs the beam at a vehicle's windshield and then lowers the beam to the front license plate in order to produce a stronger signal. The change in distance between the vehicle's windshield and its front license plate may produce a five to nine mph increase in speed. This phenomenon can be observed by moving the Lidar beam quickly from the windshield to the front license plate of a parked car. The stationary vehicle will produce a speed reading of five to nine mph.

Lidar Sight Misalignment

Law enforcement officers aim Lidar devices using a heads-up display (HUD) which contains a set of cross hairs and a digital screen that displays speed, distance, and approximate beam location. As with gun sites, HUD sites can become misaligned if dropped or mishandled. Misalignment may result in the officer measuring the wrong car.

Lidar Calibration

Every six months all Lidar devices are required to be sent to the manufacturer's laboratory to be calibrated for accuracy. Additionally, at the beginning and end of each shift the officer should perform the tests on their Lidar device. If the officer does not perform these tests and bring proof to court, the speeding ticket may be dismissed.

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